Some applications require the generation of reference voltages which are thermally compensated and have low noise. These reference voltages are typically symmetrical about an analog (VCC/2) ground. An example application is a switched-capacitor integrated circuit used in a Sigma-Delta converter.
FIG. 1 illustrates a circuit diagram of a prior art second order Sigma-Delta modulator for an analog/digital converter (A/D). VH and VL are reference voltages that define a maximum input dynamic range for the system.
FIG. 2 illustrates a switched-capacitor biquadratic cell for filtering a digital bit stream in a prior art Sigma-Delta digital/analog converter (D/A). Depending on the logical value of the bit stream (`1` or `0`), a positive voltage (VH) or a negative voltage (VL) is applied. These voltages are generated with respect to the input analog reference potential (analog ground) of the filter.
In both applications, as shown in FIGS. 1 and 2, performance of the respective A/D and D/A converters depends upon the "quality" of these reference voltages VH and VL. For instance, a noise superimposed on these voltages is translated into an error of the charge stored in the input capacitances, and hence, on the integrated value at the output of the two structures. This in turn limits the signal-to-noise ratio of these devices. Current high resolution audio converters use reference voltage sources external to the converter chip. They are typically formed on the printed circuit board using adequately filtered and compensated voltage supplies.
A fully integrated alternative approach adopted in prior art devices is illustrated in FIG. 3. Referring to FIG. 3, the reference voltages are generated from the supply voltage using a resistive divider and are buffered by low noise amplifiers. However, inaccurate voltage values are obtained and the rejection of supply noise may be ineffective. The value of an integrated resistance is defined with a precision of only about .+-.15%.
In addition, since these integrated circuits are often a mix of digital and analog components, the voltage supply lines are affected by digital noise correlated to the clock frequency of the digital circuitry. Accordingly, it is not uncommon for amplitudes of several tens of mV (RMS) of noise to be superimposed on the DC supply voltage (VCC), as well as on the reference voltages derived from it.
To filter this noise, large external capacitors (e.g., several tens of .mu.F) are normally used. However, this adds to the total cost of the application. Another drawback of this particular approach is the thermal drift of the reference voltages caused by temperature variations of the integrated resistors (polysilicon or "well" type).
Many integrated devices have circuits that generate reference voltages of adequate value either by the use of resistive voltage dividers or by the use of analog multipliers. These reference voltages originate from an on-chip generation of the bandgap voltage for the silicon (approximately 1.2-1.3 V) which is constant with temperature.
When generating symmetrical reference voltages for the peculiar applications mentioned above, their dependence on the temperature must be minimized and the rejection of noise superimposed on the supply voltage must be maximized. In addition, the voltages must not be overly sensitive to undesired conditions that may arise due to the inevitable spread of the nominal voltage values of the integrated components. Also, resistivity of interconnections may cause voltage differences due to undesired voltage drops, etc.